Re: [PATCH] dm.c - device-mapper I/O path fixes

[Denis Vlasenko]

On 11 December 2002 11:16, Kevin Corry wrote:
> > > --- diff/drivers/md/dm.c	2002-12-11 12:00:29.000000000 +0000
> > > +++ source/drivers/md/dm.c	2002-12-11 12:00:34.000000000 +0000
> > > @@ -238,10 +238,11 @@
> > >  	static spinlock_t _uptodate_lock = SPIN_LOCK_UNLOCKED;
> > >  	unsigned long flags;
> > >
> > > -	spin_lock_irqsave(&_uptodate_lock, flags);
> > > -	if (error)
> > > +	if (error) {
> > > +		spin_lock_irqsave(&_uptodate_lock, flags);
> > >  		io->error = error;
> > > -	spin_unlock_irqrestore(&_uptodate_lock, flags);
> > > +		spin_unlock_irqrestore(&_uptodate_lock, flags);
> > > +	}
> > >
> > >  	if (atomic_dec_and_test(&io->io_count)) {
> > >  		if (atomic_dec_and_test(&io->md->pending))
> >
> > This seems pointless, end result:
> >
> > 	spin_lock_irqsave(&_uptodate_lock, flags);
> >  	io->error = error;
> > 	spin_unlock_irqrestore(&_uptodate_lock, flags);
>
> Are you saying the "if (error)" part is pointless? If so, I have to

No. Locking is pointless. What exactly you try to protect here? 

> disagree. A bio may be split into several sub-bio's. When each of
> those split bio's completes, they are going to call this function.
> But if only one of those split bio's has an error, then the error
> might get lost without that "if" statement.
>
> However, it might be a good idea to consider how bio's keep track of
> errors. When a bio is created, it is marked UPTODATE. Then, if any
> part of a bio takes an error, the UPTODATE flag is turned off. When
> the whole bio completes, if the UPTODATE flag is still on, there were
> no errors during the i/o. Perhaps the "error" field in "struct dm_io"
> could be modified to use this method of error tracking? Then we could
> change dec_pending() to be something like:
>
> if (error)
> 	clear_bit(DM_IO_UPTODATE, &io->error);
>
> with a "set_bit(DM_IO_UPTODATE, &ci.io->error);" in __bio_split().

You seem to overestimate my knowledge of this part of the kernel. :(
--
vda